This invention relates to an impedance measuring device, especially applied to a capacitance measurement of a digital multimeter.
For a capacitance measuring device of digital multimeter, the following measurement method has been proposed. This device comprises a CR differentiation circuit for differentiating a squarewave signal and providing a differentiated pulse, and a voltmeter circuit for measuring and displaying a capacitance value in accordance with the average voltage level of the differentiated pulse. The basic principle of this measurement method may be considered as follows. Now suppose that the amplitude of an input squarewave signal is Ei, the period thereof is T and the average voltage level of a differentiated pulse obtained from the CR differentiation circuit is L.sub.ave. Then, the following expression is applicable: ##EQU1## where t denotes a time. In equation (1A), assume that ##EQU2## From the assumption or approximation of equation (2A), equation (1A) can be simplified as: ##EQU3## In equation (3A), when the parameters, T, Ei and R have known constant values, the average level L.sub.ave can be regarded as proportional to the capacitance C.
Here realization of assumption (2A) is required for establishing the proportional or linear relation between L.sub.ave and C. This requires that the pulse area of the differentiated pulse be extremely small, or voltage magnitude of L.sub.ave be little, in comparison with the input squarewave signal. By the measurement method based on equation (1A) therefore a very high-sensitivity voltmeter, generally a DC voltmeter, must be used. For this reason, a digital multimeter being applied with the above-mentioned measuring method is liable to be affected by DC-level variation occurring at the time of measuring range change. Thus, a zero-adjust operation is required by the user when a capacitance measuring is performed.